An amplifier is an electronic device that can increase the power of a signal by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude. In this sense, an amplifier can modulate the output of the power supply to make the output signal stronger than the input signal. An amplifier is effectively the opposite of an attenuator: while an amplifier provides gain, an attenuator provides loss.
An amplifier can either be a separate piece of equipment or an electrical circuit within another device. The ability to amplify is fundamental to modern electronics, and amplifiers are widely used in electronic equipment. The types of amplifiers can be categorized in different ways. One is by the frequency of the electronic signal being amplified: audio amplifiers can amplify signals in the audio (sound) range of less than 20 kHz, and radio frequency (RF) amplifiers can amplify frequencies in the radio frequency range between 20 kHz and 300 GHz. Another consideration is which quantity, voltage or current, is being amplified. Therefore, amplifiers can be divided into voltage amplifiers, circuit amplifiers, transconductance amplifiers, and transresistance amplifiers.
A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Some transistors are packaged individually, but many are found embedded in integrated circuits.
Transistors are utilized for their ability to use a small signal applied between one pair of its terminals to control a much larger signal at another pair of terminals. This property is called gain. It can produce a stronger output signal, a voltage, or current, which is proportional to a weaker input signal; that is, it can act as an amplifier. Alternatively, the transistor can be used to turn current on or off in a circuit as an electrically controlled switch, where the amount of current is determined by other circuit elements.
The above-described background relating to amplifiers is merely intended to provide a contextual overview of transistors, and is not intended to be exhaustive. Other context regarding amplifiers may become further apparent upon review of the following summary and detailed description.